The notion that slavery was just a side issue is amazingly weird and counter-historical.
In the Lincoln-Douglas debates, 21 hours if I remember correctly between two of the most effective politicians in America, they talked about almost nothing else. Odd for a backburner issue.
In Lincoln’s 1st Inaugural, he talked mostly about slavery and the possibly approaching war. Strange if slavery wasn’t really an issue.
The Democratic Party split, twice, in 1860 specifically over the issue of whether a federal slave code should be imposed on the territories. But that was really a minor issue compared to tariffs and such.
In Bleeding Kansas they were really shooting at each other over tariff rates. John Brown hacked up five proslavery settlers encourage passage of the Morrill Tariff.
I believe every single state that seceded proclaimed the protection of slavery to be its primary reason. Several, I believe, mentioned nothing else. But of course there were really other, unmentioned, far more important reasons behind secession and war.
/s
All good points. After the Civil War, with slavery discredited, Southern partisans were at pains to find other grounds on which to justify the catastrophe of the secession and ensuing war. As with the South’s prewar efforts to justify slavery, there was an air of unreality to these explanations and they do not bear up under scrutiny.